1. Field of the Invention
The present invention relates to a making method for a seamless metallic tube, and more specifically relates to a piercing rolling method for a seamless metallic tube with a tilting roll type piercing rolling mill.
2. Description of the Related Art
In a Mannesmann tube-making method, which has been widely used in a making method for a seamless metallic tube, a solid round billet (hereinafter referred to as only “billet”) heated at a desired temperature, which is used as a raw material, is supplied to a tilting roll type piercing rolling mill (hereinafter referred to as only “piercer”) to pierce a hole in the axis center portion thereby obtaining a hollow tube stock.
Then, the obtained hollow tube stock is stretching rolled with a subsequent stretch roll mill, such as a plug mill, a mandrel mill or the like, as it is or after optionally setting the diameter of the tube stock by enlarging or reducing the diameter of tube stock by passing the hollow tube stock to an elongater mill or a shell sizer having the same configuration as said piercer. Then it is subjected to a refining process including tube-polishing, shape-correcting or sizing with finishing roll mills such as a stretch reducer, a reeler, a sizer and the like to make a product tube.
FIG. 1 is a perspective view showing a configuration example of a piercer used in a Mannesmann tube-making method. The piercer is constructed so that it includes a pair of barrel type main rolls 1, 1 oppositely disposed while being inclined to the opposite direction each other with axis-symmetrically arranging to a pass line X—X, which is a supply line for a billet 4 that is a material to be pierced, and further includes a pair of disk rolls 2, 2 oppositely disposed with axis-symmetrically arranging to said pass line while the phases of the disk rolls are differentiated from those of these main rolls 1, 1 by 90° as well as a plug 3 is supported on the pass line X—X with a mandrel.
The nose (tip) of the plug 3 is usually disposed such that it is positioned at a rolling upstream side than a gorge 6 where the distance between the main rolls 1, 1 is minimum, and a distance (for example, PL shown in FIG. 4, which will be described later) protruded from the gorge 6 is called as a plug lead.
In the piercer constructed as mentioned above, the main rolls 1, 1 are rotated in the same direction with an inclination angle β with respect to the pass line X—X. Consequently, the billet 4 supplied in an arrow direction along the pass line X—X is moved spirally after engagement between the main rolls 1, 1 so that it is hollowed at the axis center portion of the billet to obtain a hollow tube stock.
In the step the disk rolls 2, 2 act as a guide member of the rolling billet 4 and at the same time act an outer diameter-shape corrector by suppressing the bulging of the hollow tube stock pierced by the plug 3 in a 90° phase direction with an opposite direction of the main rolls 1, 1. Further, these disc rolls 2, 2 are rotation-driven in the same direction as a billet 4 feed direction so that sliding between pierced hollow tube stock and the rolls is reduced and no scoring occurs.
Further, the piercers include a piercer, whose main rolls 1, 1 each has a cone type shape, called as an intersection type one, which forms an intersection angle γ, which is different from the above-mentioned inclination angle β by disposing the roll axis center so that it is closer on the inlet side and farther on the outlet side with respect to the pass line X—X (refer to FIG. 11B, which will be shown later).
In recent years even in a material having less workability such as a high alloy steel, stainless steel or the like, rolling of a metallic tube has been performed by use of Mannesmann tube-making method. Therefore, the above-mentioned plug 3 is strongly required for performance of a long service life and performance that inside defects are not generated in the hollow tube stock.
To suppress the inside defects, which are generated in the hollow tube stock, it is indispensable to suppress (a) the generation of rotary forging effect and (b) the generation of circumferential shearing strain as described in Japanese Patent Application Publication No. 57-168711. These phenomena of (a) and (b) are peculiar phenomena of a piercer. Thus as long as these phenomena are suppressed, a material having less workability such as a high alloy steel, stainless steel or the like cannot be worked to tubes efficiently by the Mannesmann tube-making method. Further extension of the service life of a plug used is also difficult.
The above-mentioned Japanese Patent Application Publication No. 57-168711 discloses a method of suppressing the above mentioned (a) and (b) by controlling the inclination angle β and intersecting angle γ. However, in the publication not only an elongation of the service life of the plug but to cause the plug itself to have functions of suppressing the above-mentioned (a) and (b) are not considered at all.
Further, Japanese Patent Application Publication No. 10-137818 proposes a plug shape by which a service life can be extended even if a plug is used in piercing rolling of a material having less workability such as a high alloy steel, stainless steel or the like. FIG. 2 is a view showing plug shapes proposed in Japanese Patent Application Publication No. 10-137818.
As shown in FIG. 2, the proposed plug is a plug called as a shell-shaped plug, whose entire shape is simple, so called as 2-zone type plug (hereinafter referred to as “2-zone type plug”). The relationships between only r, R and D of the sizes of respective portions of the plug shown in FIG. 2 were defined as plugs of shapes, which satisfy the conditions shown in the following expressions (5) to (7). Thus, to cause the plug itself to have functions of suppressing the above-mentioned (a) and (b) is not considered at all.R≧−160r+12D  (5)R≧18r+3.6D  (6)−20r+22D≧R≧90r−15D  (7)
FIG. 3 is a view showing another plug shape proposed as a plug of a long service life. This plug has been proposed by a Germany reference (by Neumann “Stahlrohrnerstellung (production of steel tube; German reference)”, 1970) and has a structure in which between a front end portion having a curvature radius of r and an axial length of L1 and a work portion of an axial length L3, which is an arc rotating surface of a curvature radius of R, was formed a cylindrical parallel portion having an outer diameter of d and an axial length of L2 and an front end rolling portion comprising this parallel portion and said front end portion was formed.
Since the plug having a shape shown in FIG. 3 has such a structure that a gap where a material to be pierced does not contact the vicinity portion of the work portion in the front end rolling portion is formed and heat accumulated within the plug is discharged, the tip of the plug is difficult to dissolve thereby extending the service life of the plug.
Thus, the present inventors performed use comparison tests between said 2-zone type plug shown in FIG. 2 and the plug having the shape shown in FIG. 3. As a result it has been confirmed that the plug having the shape shown in FIG. 3 has a slightly longer service life and inside defects, which are more difficult to occur than the other, but there are problems that uncompleted engagement is liable to occur and reducibility is reduced.